You don’t want the floss just moving in and out between the teeth. The point of floss is to scrub the inside face of each tooth. So when the floss is between 2

You don’t want the floss just moving in and out between the teeth. The point of floss is to scrub the inside face of each tooth. So when the floss is between 2 teeth you want to press it against first one tooth and move it up and down several times and then press it up against the other tooth and do the same thing. There’s really no way to do that to multiple teeth at once without something much much more complicated.